Foreign object debris is a substance, debris, or article alien to a product or system that would potentially cause damage, or malfunction. Foreign object damage is any damage attributed to a foreign object, that is, any object that is not part of the product, that can be expressed in physical or economic terms and may or may not degrade the product's required safety or performance characteristics. FOD is an acronym often used, particularly in aviation, to describe both the damage done to products by foreign objects, and the foreign objects themselves. In the aviation industry alone, it is believed that foreign object damage causes as much as thirteen billion dollars per year in direct and indirect costs, including delays, aircraft changes, incurred fuel costs, and unscheduled maintenance.